Differential refractometer cell



J1me 1953 F. w. CRAWFORD 2,837,007

DIFFERENTIAL REFRACTOMETER CELL Original Filed Sept. 29, 1950 3 Sheets-Sheet 1 2 ]N V EN TOR. F. W. CRAWFORD ATFORNE June 1953 F. w. cRAwFoRb 2,837,007

DIFFERENTIAL REFRACTOMETER CELL Original Filed Sept. 29, 1950 5 Sheets-Sheet 2 -B f I A omv s June 3, 1958 F. w. CRAWFORD 2,337,007

DIFFERENTIAL REFRACTOMETER CELL Original Filed Sept. 29, 1950 s Sheets-Sheet 3 AMPUFIER 3:6 "1 E;

49- i.=-so

LL INVENTOR. |7e 17i F.W. CRAWFORD H LW z ATTZNEZ United States DIFFERENTIAL REFRACTOMETER can Francis W. Crawford, Idaho Falls, id-also, assignorfo Phillips Petroleum Company, a corporation of Betaware 7 Claims. (Cl. 88-14) This invention relates to apparatus for analyzing a desired process or sample stream. in one specific aspect it relates to a. split cell assembly for use with a differential refractometer. In another specific aspect it relates to apparatus for maintaining equal temperatures and pressures at both cells of a differential refractometer.

This application is a divisional application of my copending application Serial No. 187,600, filed September 29, 1950, now Patent No. 2,724,304.

l-leretofore, considerable difficulty has been experienced in providing a simple, reliable mechanism for indicating the composition of process streams, and for the in dication and control of various process variables. In accordance with this invention, 1 provide such apparatus at very low cost by utilizing the principle of difierential refraction whereby the refractive index of a sample or stream to be analyzed is compared with the refractive index of a standard fluid or with that of a second sample or stream, the apparatus automatically indicating the difference in refractive index, which is a measure of the relative compositions of the two streams or samples. This difference in refractive index may be recorded and, for this purpose, I provide a novel circuit for controlling the range and sensitivity of the recorder so that either small or large changes in relative refractive index may produce full scale deviations of the recorder pen. Manifestly, the diiference in refractive index may be utilized in many automatic control operations, such as control of stabilizing column in a refinery by automatic variation of heat supplied to the column, the reflux atio, the feed rate, or other process variable. tus of my invention is useful in either indicating or automatic control operations wherever variations in the com position of a stream produce detectable changes in its refractive index. I also provide means for effecting a close control of the temperature and pressure of fluids passed to the refractometer so that such variations do not affect the output of the apparatus.

It is an object of this invention to provide an improved continuous analyzing device utilizing the principle of dif terential refraction.

It is another object to provide an improved split cell for use in such a device.

It is a further object to provide means for effecting close control of temperature and pressure in such a device.

It is a still further object to provide apparatus which is rugged in construction, durable and economical to build and manufacture.

Various other objects, advantages and features of the invention will become apparent from the following tailed description taken in conjunction with the accompanying drawings, in which:

Figure l is a top view of the refractometer of my invention;

Figure 2 is a bottom view of the apparatus of Figure 1;

Figure 3 is a detail view of the differential refractom- In fact, the apparai 1 atent Q 2,837,007 Patented June 3, 1958 eter cell together with a portion of the temperature and pressure regulating device.

Figure 4 is a schematic circuit diagram of the refractometer.

Figure 5 is a vertical sectional view of a modified form of refractometer cell;

Figure 6 is a schematic view of an optical system utilizing the cell of Figure 5; and

Figure 7 is a schematic circuit diagram of a modified photoelectric cell circuit.

Referring now to Figure l, the apparatus includes a base 10 upon which is mounted a radiation source 11 to direct a radiation beam through an adjustable coupling or bellows 12 to a difierential refractometer cell 13, the beam being displaced in cell 13 by an amount which is proportional to the difference in refractive index of the two fluids contained therein. After leaving the cell 13, the beam is reflected by a movable mirror assembly 14 and stationary mirror assemblies 15, 16 to a dual or twin radiation detector 17. The beam is adjusted automatically by movement of mirror assembly 14 until it is centered upon the dual detector assembly 17, the amount of rotation of assembly 14 necessary to effect this adjustment being a measure of the difference in refractive in- (16X of the two fluids within cell 13.

in a preferred embodiment of the invention, the source 11 is an ordinary incandescent bulb producing visible light which is mounted within a housing 11a, this housing having a cooling coil or pipe 11b disposed in heat transfer relation therewith to dissipate the heat produced by the bulb. Light produced by the bulb is directed through the tube and, thence, through bellows 12 to the cell 13, the bellows having a collar 12a at one end thereof which is connected to tube 110 and having a collar 12b at its other end which is connected to a flange collar 13a, Figure 3, of the cell unit 13. It will be ap parent that the bellows 12 serves as an adjustable coupling between the source 11 and the cell 13.

A lens 13b, Figure 3, which is mounted between collar 13a and an annular member 13c, as by suitable screws, one of which is shown at 13d, collirnates the light passing through the tube 11c and bellows 12 so that a beam passes through a battle 13x which prevents light from reaching the cell walls of the cell assemblies now to be described. These cell assemblies are formed from two complementary metal blocks 13c, 13 which define an interior passage closed at one end thereof by a transparent window 13g mounted between block 132 and annular member 130, this window having a sealing gasket 1311 in contact therewith to prevent escape of liquids from the interior of the cell unit. The other end of the passage is closed by a similar window 13: which is mounted between block 13 and an annular member 131', this windown also being provided with a sealing gasket 13k. Annular member 13s is secured to block 13a by a plurality of screws, one of which is shown at 13m, and, in similar fashion, annular member 13 is attached to block 13 by a plurality of screws, one of which is shown at 1311. The blocks 13:: and 13 have interfitting portions so that, when they are assembled, a rigid unitary structure is formed.

A diagonal transverse plate'13p of transparent material is mounted between the blocks 13e and 13f, this plate being provided with a sealing gasket 13:; and dividing the interior passage of the cell into two chambers Br and 13s. Chamber 13r,is provided with an inlet conduit 13f and a valved outlet conduit 1314 while chamber 13s, of slightly greater diameter, is provided with an inlet conduit 13v and a valved outlet conduit 13w.

In operation of the cell, chamber 13s is filled with a standard fluid while chamber 137' is fflled with a fluid to be analyzed which may "be continuously circulated I therethrough, if desired. Light entering the cell through collar 13a is collimated by lens 13b and passes, as a beam, through window 13g, and plate 13p, the beam then leaving the cell through window 131 and passing to the movable mirror assembly 14. If the refractive index of the liquids in the two cells is identical, the beam leaves the cell in an axial path but, if the refractive index of the liquids diflers, the beam is deviated from its axial path by an amount proportional to the difference in refractive index between the two liquids. instead of con taining a standard liquid, the chamber 131- can contain any other fluid with which it is desired to compare the refractive index of a process stream or other streams to be analyzed. For example, a second process stream can be passed continuously through cell 13;- and the deviation of the beam from its axial path is then continuously proportional to the difference in refractive index between the two streams.

It is important that a close control of temperature and pressure of the cell 13 be maintained, particularly when visible radiation is utilized, although control of these conditions is also important when ultra-violet, infra-red or other types of radiation are used. In order to regulate the pressure, that is, to maintain the same pressure in the two chambers 13r and 13s, I provide an equalizing device 18 formed from a tubular piece 18a of metal defining an interior chamber 1812 having a threaded opening 18c therein into which the conduit 13v is fitted, this chamber also communicating through a bore 18d and a nipple or fitting 182 with a valved conduit 18f. Accordingly, the pressure within chamber 18b is the same as that in chamber 13s. Mounted in the chamber 18b is a bellows 18g which communicates through a bore 18h and a coupling 181' with a valved inlet conduit 18 The interior of the bellows also communicates by a bore 18k and a fitting 18m with the inlet conduit 13! of chamber 13r.

As a result, the pressure interiorly of the bellows 18g is the same as that existing in chamber 13r. It will be apparent that the bellows 18g expands or contracts, thereby changing the relative volumes of the chambers 18h, 13r and 18b, 13s until the pressures therein are equal. Only a small variation in pressure ordinarily need be compensated for ,by the equalizer 18 and it functions 7 in a very sensitive manner to equalize the pressures in the two chambers.

Temperature control of the cellunit 13 is effected by forming the blocks 13e, 13f from a metal which is a good conductor of heat and by making these blocks sufiicie'ntly massive to serve as a reservoir thermally contacting the chambers 13r and 13s. In this connection, it will be noted that increases or decreases in the temperature of 'the cell as a whole does not have any substantial effect upon the bending of the beam passing through the cell.

However, even a small temperature variation between the I fluids in the two cells may result in errors. Thus, the described construction of unit 13, while it does not eliminate temperature variations of the whole cell, provides eflicient heat transfer characteristics such that the fluids in the two cells remain at almost exactly the same temperature.

A further improvement in the described temperature control is effected by passing the material to be analyzed through a conduit 19a which in turn passes through a rather large storage vessel 19b containing a liquid and, thence, through conduit 18 and the pressure equalizer 18 to cell 13r. Even rather large variations in the temperature of the fluid entering through line 19a produce only small changes in the temperature of the entire body of liquid within the vessel 19b with the result that the temperature variations in fluid fed to the cell unit are very small. These small variations in temperature of the fluid in chamber 13r are readily transferred to the fluid in chamber 13s by the eflicient heat transfer construction of the cell with the result that the two bodies of fluid remain accurately at the same temperature. This contributes materially to the accuracy of the analysis performed by this apparatus.

Large temperature variations are also prevented by the coil 11b which carries away most of the heat produced by the source 11 and by metal tubing 20 which is disposed in a sinuous path at the underside of the base 10,'jcooling fluid being circulated through this tube from an inlet conduit 20a and passing through the tube to an outlet conduit 205. This tends to maintain the base, and thereby the cell unit 13, at a constant temperature.

Referring now to Figure l, the movable mirror assembly 14 includes a base 14a which is pivotally mounted at 14b to the base 10, the base 14a carrying a bracket 14c to which a plane mirror 14d is adjustably secured by screws 14e, these screws permitting angular adjustment of the mirror, as desired. Integrally formed with the base 14a is an elongated cam follower arm 14f carrying a cam a roller 14g which is urged into engagement with a cam 21:: by a spring 14h. The stationary mirror assemblies 15 and 16 include brackets 15a and 16a, respectively, which are secured to the base 10 by screws 15b and 16b, these brackets carrying the respective mirrors 15c, .160 which are secured thereto by adjusting screws 15d and 16d. V

The dual detector unit 17 includes a bracket 17a secured to the base 10 by screws 17b, this bracket having a casing 17c secured thereto by adjusting screws 17d. Where visible light is utilized as the radation for purposes of analysis, the casing includes two photovoltaic cells 172, 17 mounted within an insulating sleeve 17g which, in turn, is supported within the casing 170. These cells are symmetrically disposed with respect to the beamiof radiation reflected thereon from mirror 160, mirror 15c and mirror 14d. Where infra-red radiation is utilized,-

the cells are replaced by bolometers or other suitable detectors of infra-red radiation and, of course, other types of radiation require suitable detectors therefor.

The cam 21a forms a part of a cam assembly 21 which includes a shaft 21b journalled in the base 10 and carrying a gear 210 which meshes with a pinion 22a carried by a shaft 2211, this shaft being geared to an electrical servomotor 220 mounted on heat insulating blocks, not shown,

. at the upper side of the base 10. The last-mentioned parts make up a motor assembly 22 The gear 210 also meshes with a pinion 23a carried by a shaft 23b which actuates the contactor of a potentiometer 23c mounted at the top of the base 10, these parts making up a potentiometer assembly 23. r

Referring now to Figure 4, it will be noted that the radiation detectors 17e, 17 are connected in opposition by conductors 24 and 25 so as to produce a resultant voltage proportional to the difference in intensity of the radiation beams incident upon the two detector units.

The voltage appearing between leads 24, 25 is amplified by a unit 26 whose output is fed to the motor 22c, the motor being mechanically connected, as described, to the contactor or potentiometer 230.

In the operation of the analyzer as thus far described, the mirror assemblies and detector units are so adjusted that equal amounts of light fall upon the respective detector units when there is a desired relationship between the refractive indices of the fluids in the chambers 1323' 13s of cell unit 13. Ordinarily, this relationship holds when the refractive indices of the two fluids are equal. In this condition, the resultant voltage produced by the detector units is zero, and no current is fed to motor 220, Assuming that the refractive index of the fluid in chamber 131- changes so as to shift the radiation beam in a direction such that more radiation falls upon cell 17c than upon cell 17f, a voltage of one polarity is produced by the interaction of the radiation detectors 17e, 17f. This voltage is amplified by the unit 26 and fed to motor 226. The resulting movement of motor 220, pinion 22a, gear nearest 210, and cam 21a causes arm14f to move in a clockwise direction, Figure 1, thereby producing a shiftin position of the beam such that less radiation is incident upon radiation detector 17e than upon radiation detector 17 This movement continues until equal amounts of radiation are incident upon the two detectors, at which time movements of the motor is stopped. The extent of movement of the motor 22c and gear 21c,-is, of course, proportional to the deviation of the refractive index of the fluid in cell 13r from its original or standard value. This deviation is reflected by a corresponding movement of the arm of potentiometer 230 which is connected to gear 210 by pinion 23a.

Assuming that the refractive index of the fluid in chamber 13r varies in such fashion as to cause more radiations to impinge upon detector 17f than upon detector 17c, an opposite effect occurs, the voltage produced between conductors 24, 25 being of opposite polarity and thus causing the arm 14 of assembly 14 to swing in a counterclockwise direction, Figure 1. This produces a shift of the radiation beam in a direction such that less radiation falls upon detector 17] and more radiation falls upon detector 17a, the resulting movement of gear 210, motor 220 and the arm of potentiometer 230 being proportional to the extent of deviation. It will be apparent, therefore, that the position of motor 220 and the position of the arm of potentiometer 23c is a function of the diiference in refractive index of the fluids 'in chambers 13r, 13s and the potential appearing across this potentiometer is directly representative of this difiference in refractive index.

The described system is extremely sensitive since the rotation of mirror assembly 14 produces a linear shift in the radiation beam incident upon the detector assembly 17, this linearity resulting from the fact that the mirror 14a is positioned close to the cell 13. These results are superior to moving the detector assembly itself since small movements of mirror 14d are, in eifect, amplified by a factor proportional to the length of the optical path formed by mirrors 15c and 160.

In accordance with the invention, 1 also provide a circuit for adjusting the sensitivity and range of a recorder upon which the described variations in refractive index are recorded. To this end, I provide a constant current power supply which produces a constant current through conductor 27. This current is produced by a power supply which includes a transformer 29 having its primary winding connected in circuit with an alternating current source 3% and a switch 31, the secondary winding being connected in circuit with a rectifier 32, filter impedances 33, 34, 35, 36, filter condensers 37, 38, 39 and a voltage regulator tube 41). The positive terminal 41 of this constant voltage power supply is connected to conductor 27 by a resistance 42 of high ohmic value so that a constant current flow through conductor 27.

The conductor 27 is connected through a variable resistance 43 to a lead 44, the fixed terminals of potentiometer 23c being connected to leads 28 and 44, respectively. A potentiometer 45 has its fixed terminals connected to leads 28 and 4-4, respectively, the contactor of this potentiometer being connected by a lead 46 to one input terminal of a recorder 47', the other input terminal of which is connected by a lead 48 to the contactor of potentiometer 230. The lead 44 is also connected to the arm of a switch 48a having a series of fixed terminals 49 with fixed resistances 56 connected in shunt with each pair of adjacent fixed terminals. One end of the series of fixed terminals thus formed is connected through a fixed resistance 51 to the lead 28.

In the operation of the circuit of Figure 4, potentiometer I established at conductor 46. Adjustment of the arm of switch 48:: produces changes in range or sensitivity of the recorder. In this connection, it will be noted that fixed resistors 50, 51 are of low ohmic value with respect to potentiometers 23c and 45. When the arm of the switch 43a is moved so as to decrease the resistance, in shunt with potentiometers 23c and 45 the current flow through potentiometers 23c and 45 also is decreased which results in a lower voltage drop across the end terminals of potentiometers 23c and 45. This decrease in voltage drop decreases the sensitivity of the recorder, that is, the recorder requires a greater movement of the contactor of potentiometer 23c to produce a given voltage difference between conductors it? and Conversely, when switch 48a is moved so as to increase the resistance in shunt with potentiometers 23c and 45, the current flow through potentiometers 23c and 45 also is increased which results in a greater voltage drop across the end terminals of potentiometers 23c and 45. This increase in voltage drop increases the sensitivity of the recorder, that is, the recorder requires less movement of the contactor of potentiometer 230 to produce a given voltage difference between conductors 46 and 48. Variable resistor 43, which has a resistance intermediate that of resistor 42 and potentiometers 23c, 45, is provided to obtain a fine adjustment of sensitivity. That is, adjustment of this resistance produces a small variation in the voltage at conductor 44 which, in turn, produces a small change in the sensitivity of the recorder. In effect, the variable resistor 43 provides a vernier adjustment between the steps in sensitivity obtained by the movement of the arm of switch 48a to its various positions. It will be evident, therefore, that the described circuit enables the zero point of the recorder to be adjusted to any desired point and, also, the circuit allows the sensitivity of the recorder circuits to be continuously varied within wide limits.

Referring now to Figure 5, I have shown a modified cell assembly, parts similar to the cell assembly of Figure 3 being indicated by like reference characters. In this assembly, a lens 113a is provided opposite the lens 13b, this lens being secured in position by an end cap 1131) secured to member 13j as by screws, one of which is shown at 1130. The cell assembly further includes a heat exchanger coil outside the blocks 13e and 131'", this coil being inserted at any suitable place in the conduit 20, Figure 1, so that circulation of cooling fluid therethrough tends to maintain the cell at a constant temperature. If desired, the heat exchanger coil 113d may be formed by boring suitable passages through the blocks 132 and 13 The purpose of the lens 113a, referring to Figure 6, is to produce a convergence of the light beam passing therethrough so that such beam falls upon a more restricted portion of the photovoltaic cells 17e and 17f than in Figure 4. This is well illustrated in the figure, wherein the folding of the beam produced by mirrors 15c and has been omitted for clarity, and it will be noted that the light beam extends only a short distance upon each side of the junction between the cells 17:: and 17; rather than falling upon a relatively large portion of the cells as in Figure 1. If the index of refraction of the fluid in cell 13r is greater than that of cell 13s, the beam is deflected upwardly, thereby causing the mirror to swing in a counterclockwise direction to center the beam upon the cells 17:: and 17;), such rotation of the mirror operating potentiometer 23 in the manner previously described to produce an indication representative of the difference in indices of refraction. Conversely, if the index of refraction of the fluid in cell 131' is less than that in cell 13s, the beam is deflected downwardly so that a clockwise rotation of mirror 14d is necessary to cause the beam to fall in centered relationship upon the cells 17:: and 17f, thereby moving the potentiometer 23 to a position representative of this difference in indices of refraction.

In Figure 7, I have illustrated a modification of the circuit of Figure 4 wherein photocouductive cells are substituted for the photovoltaic cells 172 and 171. These cells, which are'represented by resistances 135 and 136 in the drawing, are connected in a Wheatstone bridge circuit with balancing resistors 137 and 138, a battery 139 being connected between two opposite terminals of the bridge and the conductors 24, 25' being connected across the .opposite terminals of the bridge. This circuit is particularly suitable for use with infra red radiation wherein the cells 135, 136 can be bolometers. However, thedescribed photoconductive cells may be any other type wherein the ohmic resistance of the cell varies with changes in intensity of the radiation incident thereon.

The. circuit of Figure 7 is also suitable for use with photoemissive cells wherein 21 treated cathode mounted in a glass envelope emits electrons under the action ofradiation, these electrons being collected at an anode. Such tubes may be vacuum tubes or they may be gas filled. It

will be understood, therefore, that the resistances 135 and 136 may represent photoemissive cells.

- While the invention has been described in connection with present, preferred embodiments thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention.

I claim: I

l. A combination of differential refractometer cell and pressure controlling means, said combination comprising: a structure shaped to formwa central passage, radiation transparent means dividing said passageinto first and second chambers whereby a beam of radiation directed through said passage is deflected by an amount proportional to the difference in refractive indices of fluids positioned Within said first and second chambers, a first inlet conduit and a first outletc ond'uit communicating with one of said chambers, a second inlet conduit and a second out- -'let conduit communicating with the other of said chambers, a pressure equalizing device comprising a casing and a hollow expansible member positioned within said casing, means connecting one of said first conduitsto the interior of said casing, and means connecting the other of said first conduits to the interior of said expansible member wher by the pressures Within each of said chambers are substantially equal.

2. A combination of differential refractomcter cell and pressure controlling means, said combination comprising: a structure shaped to form a central passage, radiation transparent means dividing said passage into first and second chambers whereby a beam of radiation directed through said passage is deflected by an amount proportional to the difference in refractive indices of fluids positioned within said first and second chambers, a first inlet conduit and a first outlet conduit communicating with one of said chambers, a second inlet conduit and a second outlet conduit communicating with the other of said chambers, a pressure equalizing device comprising a casing and a hollow expansible bellows within said casing, means connecting one of said first conduits to the interior of said casing, and means connecting the other of said first conduits to the interior of said bellows whereby the pressures Within each of said chambers are substantially equal.

3. A combination of differential refractometer cell, temperature controlling mean and pressure controlling means, said combination comprising: a metal structure shaped to form a central passage, radiation transparent means dividing said passage into first and second chambers whereby a beam of radiation directed through said passage is deflected by an amount proportional to the difference in refractive indices of fluids positioned in said first and second chambers, said structure being sulficiently massive as to permit etficient heat exchange between the :fluids in said chambers, fluid inlets and outlet means communicating with each of said chambers, a vessel adapted to contain a liquid at a predetermined temperature, said vessel being of large volume as compared to the volume of said chambers, means for introducing fluid into one of said chambers through a conduit passing through said vessel whereby the fluid introduced into said one chamber is at atemperature substantially equal to said predetermined temperature, a pressure equalizing device comprising a casing and a hollow expansible member positioned within said casing, means for connecting the interior of saidcasing-w it h =one of;said chambers, and means for connecting the interior of said expansible member whereby the pressures within each of said chambers are substantially equal.

4; A combination of differential refractometercell and pressure controlling means, said combination comprising: first and second complementary blocks of heat conductive material shaped to form a central passage,.,a radiation transparent window positioned at each end ofsaid passage, an annular, plate securing each of said windows in position at the respective ends of said passage in ifluid tight engagement with said respective blocks, a bafiiepositioned adjacent the inlet end of said passage to prevent radiation transmitted through said passage from striking the walls thereof, a plate of radiation transparent material positioned between portions of said blocks to divide said passages into two chambers, said transparent plate being positioned such that the plane thereof is at an angle other than a right angle with the longitudinal axis of said pas." sage, a first inlet conduit and a first outlet conduit cornmunicating with one of said chambers, a second inlet conduit and a second outlet conduit communicating with the other of said chambers, a pressure equalizing device comprising a casing and a hollow expansible member positioned within said casing, means connecting one of said first conduits to the interior of said casing, and means connecting member of said first conduits to the interior of said expansible member, whereby the pressures within each of said chambers are substantially equal.

5. A combination of difierential refractome'ter cell and pressure controlling means, said combination comprising: first and second complementary metallic blocks shaped to form a central passage, the portion of said passage through said first block being of smaller cross-sectional area than the portion :of said passage through said second block, a radiation transparent window positioned :at each end of said passage, an annular plate securing each of said windows in position at the respective ends of said passage in fluid tight engagement with said respective blocks, a

battle positioned adjacent the portion of said passage 7 through said first block to prevent radiation-transmitted through said passage from the portion of said passage through said first block to the portion of said passage through said second block from striking the walls of said passage, a plate of radiation transparent material posi-- tioned between portions of said blocks to divide said passages into two chambers, said transparent plate being po 7 sitioned such that the plane thereof is at an angle other than a right angle with the longitudinal axis of said passage, a first inlet conduit and a first outlet conduit communicating with, one of said chambers, a second inlet conduit and a second outlet conduit communicating with the other of said chambers, a pressure equalizing device comprising a casing and a hollow expansible member positioned .Within said casing, means connecting one of said first conduits to the interior of said casing, and means connecting the other of said first conduits tothe interior of said expansible member whereby the pressures within each of said chambers are substantially equal.

6. A combination of differential refractometer cell, temperature controlling means and pressure controlling means, said combination comprising: first and second complementary metallic blocks shaped to forma central passage,-the portion of said passage through said first block being of smaller cross-sectional area than the portion of said passage through said second block, a radiation transparent window positioned at each end of said pas sage,-an annular plate securing each of said windows in position at the respective ends of said passage in fluid tight engagement with said respective blocks, a baffle positioned adjacent the portion of said passage through said first block to prevent radiation transmitted through said passage from the portion of said passage through said first block to the portion of said passage through said second block from striking the walls of said passage, a plate of radiation transparent material positioned between portions of said blocks to divide said passages into two chambers, said transparent plate being positioned such that the plane thereof is at an angle other than a right angle with the longitudinal axis of said passage, a first inlet conduit and a first outlet conduit communicating with one of said chambers, a second inlet conduit and a second outlet condui-t communicating with the other of said chambers, means for introducing fluid into one of said chambers through a conduit passing through said vessel whereby the fluid introduced into said one chamber is at a temperature substantially equal to said predetermined temperature, a pressure equalizing device comprising a casing and a hollow expansible member positioned within said casing, means for connecting the interior of said casing with one of said chambers, and means for connecting the interior of said expansible member whereby the pressures within each of said chambers are substantially equal.

7. A refractometer cell comprising an opaque body having substantially the shape of a rectangular parallelepiped and divided into two halves along a plane perpendicular to one pair of parallel faces of the parallelepiped and angularly disposed with respect to the other four faces thereof at an angle diifering from ninety degrees and extending from the exterior of one of said other four faces near one edge thereof all the way to the exterior of the opposite face near the diagonally opposite edge, a plate of transparent material disposed between said halves, each .of said body halves having a hole extending therethrough terminating at one end at said plate and at the opposite end on one of another pair of parallel faces of said parallelepiped, and transparent means closing the outer ends of said holes, the last said means and said holes and said plate providing a light passage through said cell parallel to said faces to which the dividing plane is perpendicular.

References Cited in the file of this patent UNITED STATES PATENTS 830,225 Haber Sept. 4, 1906 2,427,996 Seaman Sept. 23, 1947 2,583,973 Stamm et al. Jan. 29, 1952 2,612,814 Glasser Oct. 7, 1952 2,624,014 Barstow Dec. 30, 1952 FOREIGN PATENTS 118,381 Sweden Mar. 18, 1947 OTHER REFERENCES Zaukelies: article on Recording Differential Refractometry in Analytical Chemistry, volume 21, June 1949, pages 743 to 745. 

7. A REFRACTOMETER CELL COMPRISING AN OPAQUE BODY HAVING SUBSTANTIALLY THE SHAPE OF A RECTANGULAR PARALLELEPIPED AND DIVIDED INTO TWO HALVES ALONG A PLANE PERPENDICULAR TO ONE PAIR PF PARALLEL FACES OF THE PARALLELEPIPED AND ANGULARLY DISPOSED WITH RESPECT TO THE OTHER FOUR FACES THEREOF AT AN ANGLE DIFFERING FROM NINETY DEGREES AND EXTENDING FROM THE EXTERIOR OF ONE OF SAID OTHER FOUR FACES NEAR ONE EDGE THEREOF ALL THE WAY TO THE EXTERIOR OF THE OPPOSITE FACE, NEAR THE DIAGONALLY OPPOSITE EDGE, A PLATE OF TRANSPARENT MNATERIAL DISPOSED BETWEEN SAID HALVES. EACH OF SAID BODY HALVES HAVING A HOLE EXTENDING THERETHROUGH TERMINATING AT ONE END AT SAID PLATE AND AT THE OPPOSITE END ON ONE OF ANOTHER PAIR OF PARALLEL FACES OF SAID PARALLELEPIPED, AND TRANSPARENT MEANS CLOSING THE OUTER ENDS OF SAID HOLES, THE LAST SAID MEANS AND SAID HOLES, AND SAID PLATE PROVIDING A LIGHT PASSAGE THROUGH SAID CELL PARALLEL TO SAID FACES TO WHICH THE DIVIDING PLANE IS PERPENDICULAR. 